To crack on is to continue doing something as quickly as possible. The phrase is also said to encourage someone to do something or start doing something.
We need to crack on and get today's work finished.
A: Can I start moving the boxes into the living room?
B: Yes, crack on.
Crack on is often followed by the preposition 'with'.
I'm going to crack on with my report.
After lunch, they cracked on with painting the kitchen.
During long periods of dry weather, earth which contains lots of clay can crack, as in the picture above. Clay gets bigger when wet and smaller when dry.